1. Technical Field
The present invention is directed to pyrolytic carbon coated carbon fiber reinforced carbon composite and useful as components for pulling single crystal apparatus.
2. Description of the Related Art
FIG. 1 shows a pulling single crystal apparatus used in the Czochralski process for manufacturing a single crystal ingot for use as a material of the semiconductor wafer and the like (hereinafter it is referred to as the CZ apparatus). As shown in FIG. 1, the CZ apparatus is so structured that a raw material in a quartz crucible 4 is heated to a high temperature by a heater 7 disposed around the quartz crucible 4 so that the raw material can be converted into the melt 5 which is pulled under reduced pressure to form the single crystal ingot 3.
The structural elements, such as a crucible 8 supporting the quartz crucible 4 and an upper ring 12, a lower ring 9 and an inner shield 11 which are subjected to radiant heat of the heater 7, are exposed to high temperature when pulling a single crystal ingot 3 from the quartz crucible 4 within a molten silicon. Accordingly, the structural elements must be formed of a material that can maintain a prescribed mechanical strength under high temperature. Further, the structural elements must be formed of a material of high-purity, because impurities, such as metals, contained in the structural elements become a cause of crystal defects in orientation of the solidifying of the single crystal ingot 3 and also become a factor of reduction of purity, when leaked during manufacturing. In general, a high purity graphite having excellent mechanical properties at high temperature and having high-purity is used for the structural elements of the CZ apparatus (Japanese Patent Publication No. Hei 6(1994)-35325). Recently, with increasing diameter of the single crystal, the single crystal pulling apparatus used in the CZ process is increased in size. This produces a handling problem caused by the increased weights for the existing graphite elements and a problem of reduction in effective processing size of the inside of the apparatus.
The carbon fiber reinforced carbon composite (C/C composite) has the properties of lightweight and excellent mechanical strength, as compared with the graphite material. By virtue of this, even when reducing in thickness, the structural elements of the C/C composite can have a strength equal to those of the graphite material, to enable an effective use of a processing chamber of the apparatus. In addition, by virtue of being lightweight, a good handling can be achieved in, for example, placement in the apparatus. By virtue of these, the crucible components used in the CZ apparatus having a large diameter are now moving from those made of the graphite to those made of the C/C composite.
Recently, the elements for use in the CZ apparatus made of a high purity C/C composite have been proposed. For example, U.S. Pat. Nos. 5,683,281 and No. 5,800,924 disclose a high purity C/C composite subjected to degasification at 2,400xc2x0 C. to 3,000xc2x0 C. under an inert gas atmosphere in its carbon fiber stage; impregnation with high purity pitch or resin for densification; graphitization; and high purification at 2,400xc2x0 C. to 3,000xc2x0 C. under a halogen gas atmosphere, so that the metal impurities of Ag, Al, Ba, Be, Ca, Cd, Co, Cr, Cu, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sr and Zn can be reduced below the detection limit by inductively coupled plasma spectroscopy (ICP). U.S. Pat. No. 5,616,175 discloses a 3-D C/C composite for pulling single crystal apparatus which is impregnated with the carbon material which results in a matrix and then graphitized, followed by high purification under a halogen gas, so that the impurities can be not more than 10 ppm and whose surface is coated with pyrolytic carbon or silicon carbide, for in preventing its reaction with the gas generated from melt 5 when the single crystal is pulled.
These prior invention disclose that the carbon fibers forming the C/C composite are subjected to the high purification process so that the impurity gas released from the inside can be minimized to achieve the C/C composite applicable to manufacturing of semiconductor or the C/C composite for the CZ apparatus, in particular.
However, in order to achieve the C/C composite applicable to the CZ apparatus, the C/C composite must be purified for a long time at high temperature over 2,000xc2x0 C. under a halogen gas atmosphere. Recently, with increasing diameter of the single crystal ingot, the CZ apparatus is increased in size, which in turn increases the size of the C/C composite. With this recent trend, the need is arising for an expensive and large-sized furnace for the C/C composite to be high purified and for more process time. This inevitably involves increase in manufacturing costs.
It is the object of the present invention to provide a C/C composite, which has such purity as to be applicable to the CZ apparatus, and whose surface is coated with pyrolytic carbon so that process time and manufacturing costs can be reduced.
To accomplish the above the object, the inventors devoted themselves on the concerned study and discovered that when the whole surface of the C/C composite is coated with pyrolytic carbon with a predetermined thickness or more, impurity gas generated from a pyrolytic-carbon-uncoated inside of the C/C composite can be shielded in a pyrolytic-carbon-coated inside thereof to achieve the applicability of the C/C composite to the CZ apparatus. Based on this discovery, the inventors have completed the present invention.
A pyrolytic-carbon-coated C/C composite of the present invention is characterized in that a total ash content of the coated pyrolytic carbon is not more than 5 ppm and a total ash content of the pyrolytic-carbon-uncoated inside of the C/C composite is 5 to 100 ppm.